The hotkeys run a script so it is possible to modify how they work. You can e.g. create a new hotkey script which just toggles a specific window. E.g. something like this:
local window = "Year Calendar"
if Rainlendar_IsWindowVisible(window) then
Create a new file called customhotkeys.lua to Rainlendar's scripts folder and add the above function to it. After you restart Rainlendar you should be able to see the new hotkey option in the settings which will toggle the year calendar.